Jenkins plugins are packaged and distributed as .hpi files (Hudson/Jenkins Plugin Interface). To manually install a plugin, an administrator must obtain this .hpi file. The installation is performed via the Jenkins web UI by navigating to "Manage Jenkins" > "Manage Plugins" > "Advanced" tab. In the "Deploy Plugin" section (previously "Upload Plugin"), the administrator can upload the .hpi file, after which a Jenkins restart is typically required to activate the plugin. This method is used when the Jenkins instance cannot access the public update center or for installing custom-developed plugins.

A. A .conf file is a generic configuration file format and is not used for packaging Jenkins plugins.

B. A .war (Web Application Archive) file is used to package an entire Java web application, such as Jenkins itself, not individual plugins.

C. A .zip file is a generic archive format; while a .hpi file is technically a ZIP archive, Jenkins specifically requires the .hpi extension for recognition.

1. Jenkins User Documentation, "Managing Plugins," section "Advanced installation." The documentation states: "If you have a plugin in .hpi format, you can install it by uploading the file from this section." (Source: jenkins.io/doc/book/managing/plugins/)

2. CloudBees, "How to install a plugin in Jenkins," section "Advanced installation." This official vendor documentation details the process of uploading a .hpi file through the Advanced tab in the plugin manager. (Source: cloudbees.com/blog/how-to-install-a-plugin-in-jenkins)